Permeable Cements

ABSTRACT

A permeable cement composition including an aqueous slurry of a hydraulic cement which is based upon a water-immiscible dispersed fluid phase and a hollow particulate material. The hollow particulate material breaks down in the presence of the cement so as to leave voids which together with the dispersed phase create a permeable structure in the cement.

The present invention relates to permeable cement compositions, inparticular permeable cement compositions for use in oil or gas wells, orthe like.

In many oil well applications, cement is used both to secure a casingwithin the well and to provide zonal isolation between the differentfluid-producing layers in the formation through which the well passes.In such cases, it is desirable for the cement to have very lowpermeability (permeability is the property of a material to allow afluid to pass therethrough, typically measured in darcies “d” ormillidarcies “md” and relative to a given fluid, e.g. oil or water).Fluid communication between the formation and the well is obtained byuse of perforating charges placed next to the producing zones ofinterest. However, there are certain applications in which permeablecements are considered useful. Typically, these are regions in which itis only required to provide mechanical support to the formation orcasing/liner near the well while still allowing flow from the formationinto the well. One such example is where the producing zone comprises apoorly consolidated formation which requires stabilization.

There have been various proposals to provide permeable cements for wellapplications. One such proposal describes the use of a mixture of sand,cement and water suspended in a hydrocarbon for injection into the zoneto be treated, see for example U.S. Pat. No. 3,429, 373; U.S. Pat. No.3,646,998; U.S. Pat. No. 3,654,990; U.S. Pat. No. 3,654, 991; U.S. Pat.No. 3,654, 992; U.S. Pat. No. 3,768,561; U.S. Pat. No. 3,861,467; U.S.Pat. No. 3,862,663; and U.S. Pat. No. 3,948,672. Another proposal is theuse of a cement incorporating a material that can be removed to leaveporosity and permeability, such as by dissolution with acid ordecomposition by heat, see for example RU 2019689 and U.S. Pat. No.6,202,751. A yet further prior proposal is to use foamed cements, oftenwith extreme foam qualities (foam quality is the ratio of the dispersedphase (gas) volume to total volume of foamed composition under givenpressure and temperature conditions). Examples of such a proposal can befound in U.S. Pat. No. 5,339,902.

Each of the prior proposals has one or more disadvantages. These includedifficulty in producing a pumpable slurry, low compressive strength ofthe set cement, the requirement for post-placement treatments ordifficulty in providing or maintaining foams at high downhole pressuresor at high foam qualities.

It is an object of the present invention to provide permeable cementcompositions that obviate or mitigate some or all of these problems.

One aspect of the invention comprises a permeable cement composition,comprising an aqueous slurry of a hydraulic cement including awater-immiscible dispersed fluid phase and a hollow particulatematerial, the hollow particulate material being one which breaks down inthe presence of the cement so as to leave voids which together with thedispersed phase create a permeable structure in the cement.

The use of a hollow particulate material, such as micro-spheres orcenospheres, allows development of a permeable structure while avoidingthe use of very high quantities of the dispersed phase, such as highfoam qualities, which can make the slurry difficult to form or place.Such materials, typically formed from aluminosilicate or other glasslike materials, break down due to the chemical and thermal environmentin the setting cement. In breaking down these materials “release” theirporosity and assist in interconnecting the dispersed fluid phase. Theexact nature and amount of hollow material used depends uponrequirements. A typical material can comprise hollow spheres havingaverage sizes in the range 350-50 microns and can be present in amountsof between 10% and 60% by volume of the dry materials used to form theslurry. A suitable source of such hollow materials is the SL range ofE-Spheres products from Envirospheres Pty Ltd of Australia. These areavailable in various size grades, for example 56 micron, 100-180 micron,290 micron and 320 micron.

The water-immiscible dispersed fluid phase can comprise gas such as airor nitrogen, or liquids such as oil. In one embodiment of the invention,the dispersed phase is a gas and the cement slurry is formed as a foam.In another embodiment, the dispersed phase is an oil present as anemulsion in the aqueous slurry.

The hydraulic cement is preferably Portland cement which may or may notinclude pozzolanic material such as blast furnace slag or fly ash ornatural materials such as pozzolana or calcined clays. It is alsopossible to use high-alumina cements also known as Ciment Fondu,plaster, Sorel cement, activated pozzolanic cements or any otherhydraulic cement.

Preferably the solid particles used to make the slurry comprise coarseparticles having a particle size in the range 100-800 microns, mediumparticles having particle sizes in the range 20-60 microns, and fineparticles having particle sizes in the range 0.1-10 micron. Among thesenumerous possibilities, a combination comprising four types of particle,namely a Portland cement, fine particles with a mean size of about 3microns, hollow cenospheres, and very coarse particles (more than 200microns) is one preferred case. However, systems using only coarse andfine particle sizes or medium and fine particle sizes or coarse andmedium particles are also possible. The fine particles can beconstituted by a micro-cement in some cases and a large particle sizecement can comprise some or part of the coarse particles if required.The hollow particles are typically in the coarse particle range but canalso fall in the medium particle size range.

Depending on requirements, different known additives can be added to theslurry such as dispersing agents, retarding agents or accelerators, orfluid loss control agents. In the case of a Portland cement, when thetemperature is over 105° C., silica is added in a proportion of 30% to50% by weight relative to the cement, as is known in the art to preventretrogression of compressive strength.

The coarse particles (e.g. hollow spheres, haematite, large particlesize cement, etc.) can be present in an amount of up to 80% by volume,preferably 30%-80% by volume, more preferably 30%-60% by volume, of thesolid materials used to make the slurry. The medium particles (e.g.Portland class G cement, hollow spheres, etc.) can be present in anamount of up to 90% by volume, preferably 10%-90% by volume, morepreferably 30%-60% by volume, of the solid materials used to make theslurry. The fine particles (e.g. Portland micro-cement, slag, etc.) arepreferably present in an amount of 0%-40% by volume, more preferablyabout 10% by volume, of the solid materials used to make the slurry.

The liquid fraction of the slurry is typically in the range 38%-50% byvolume of the slurry when the dispersed phase is a gas. When thedispersed phase is oil, the water fraction can comprise 38%-50% byvolume of the water and solids combined and the oil 40%-60% by volume ofthe total.

In the case of an oil emulsion, the use of degradable surfactants can beused to mitigate some of the detrimental effects on the cement settingmechanism. Suitable degradable surfactants are, for example, ethoxylatedcastor oil surfactants of various ethoxy chain lengths such as thosesold by Akzo Nobel under the Berol trade name, e.g. Berol 108, B27, 829and 192 (in order of decreasing chain length).

Also falling within the scope of the invention are the followingremedial applications of the cement compositions described above:

Stopping sand production from a well. When the reservoir is constitutedby unconsolidated or poorly consolidated sand, or by poorly boundsandstone or where the formation matrix can be easily dissolved bywater, fluid flow into the well entrains grains of sand. The productionof sand can lead to a stoppage in well production. To avoid such asituation, a gravel pack or filter screens are normally installed insidethe well to stop the sand. Over time, these can become soiled or blockedwith fine particles and replacing them is difficult and expensive.Placing a permeable cement according to the invention behind the casingand producing the formation fluids through this layer can prevent sandproduction into the well and prevent it from becoming necessary toperform the standard operations.

The permeable cement compositions of the invention have the advantageover conventional solutions in that a permeable pack can be made withoutrequiring fluid leakoff to concentrate a solid slurry.

When gravel-packing (or frac-packing) long intervals and/orheterogeneous permeability formations excessive leakoff may occur incertain areas leading to premature screen out and incomplete packing ofthe annulus. This has previously been overcome by mechanical means(shunt tubes). Permeable cement according to the invention will providea chemical means to do the same job by being able to form apermeable-pack without requiring leakoff and hence avoiding thepotential for screen-out and the need for mechanical placementtechnologies that anticipate this and provide alternative flow paths.

In some sand-producing formations, where some form of sand control wasnot used initially, hydrocarbon production has led to caverns/voidsbeing formed behind casing. The size of the caverns is unknown (no toolsare available to measure the size). In such a situation, to stop furtherformation sand production and/or to support the casing, a conventionalmethod would be to pump curable resin-coated proppant slurry into thecavern. Ideally with leak off this would concentrate the slurry forminga permeable consolidated pack in the cavern. However, as the size of thecavern is unknown it is impossible to design a treatment that wouldguarantee that the entire cavern would be filled. For example if theleakoff rate is higher than expected then the slurry could dehydrate tooquickly causing an early screen out perhaps leaving the cavern only halffilled. However, with permeable cement according to this invention, itis possible to continue pumping to fill the entire cavern. If the cementdisplaced reservoir fluid back into the formation quickly in one area assoon as the cement reached the rock face it would not go any further.The slurry behind is still pumpable (not dehydrated) so that continuedpumping would force the cement to fill the entire cavern.

Simple replacement of the cement behind the casing. This can be carriedout with a conventional cement, but then the casing and the new layer ofcement needs to be perforated again to re-connect the well to thereservoir, with the risk of the connection not being made if the cementlayer is thicker than planned or perforation is not deep enough.Injecting a permeable cement according to the invention avoids the needfor perforation and typically costs less and guarantees that connectionwill be established in all cases.

Applications of the permeable cement compositions in primary cementingoperations are also possible, as follows:

When the strength of the formation is sufficient to allow it, or whenthe well is highly deviated the cost and/or difficulty related toeffectively placing a casing are avoided by carrying out what is knownas an uncased or barefoot completion. Sand packing or gravel packing canthen be installed if the reservoir might produce sand. A pre-perforatedblank liner can also be employed. Placing a layer of permeable cement atthe surface of the well walls can avoid the need for such operations.Furthermore, when the formation of the reservoir is not sufficientlystrong for that type of simplified completion and it would otherwisehave been necessary to put a casing into position with cementing betweenthe casing and the well wall, putting the permeable cement of theinvention into position can reinforce the sides of the well and canavoid the need for a casing and subsequent perforation thereof.

When completion with a casing is unavoidable because the rock of thereservoir is very poorly consolidated, placing the permeable cement ofthe invention between the casing and the sides of the well, instead of aconventional cement, can avoid the need for installing gravel packing orsand packing which is very expensive.

The invention will now be described by way of the following non-limitingexamples:

EXAMPLE 1

Slurry A (reference): This slurry corresponds to the prior art. Itcomprises a class G cement and water such that the density of the slurryis 1900 kg/m³ (15.8 pounds per gallon, ppg). The slurry is foamed afteradding surfactants (aqueous mixture of polyglycols, oxyalkylates andmethanol; and a mixture of ethanol, 2 butoxyethanol and ammonium alcoholethoxysulfate—chain length 6-10) in an amount of 0.084 gallons per sack(0.007 l/kg) of powder, to obtain a foam quality of 40% (i.e. the volumeof the foam represented 40% of the final volume of the foamed slurry).

The compressive strength and water permeability are measured usingsamples which are left at ambient temperature and at atmosphericpressure for 48 hours (h) then in an oven at 85° C. for 5 days. Thecompressive strength is expressed in MPa, with pounds per square inch(psi) in brackets.

The properties of a slurry prepared in accordance with the invention ispresented and compared:

Slurry B: A mixture of powders is prepared comprising 30% by volume ofhaematite particles with a mean size of about 300 microns; 30% by volumeof hollow cenospheres with a mean size of 180 microns; 30% by volume ofclass G Portland cement and 10% by volume of a Portland/slagmicro-cement with a mean size of about 3 microns. Water and apolynaphthalene sulfonate-based super-plasticizer in an amount of 0.07gallons per sack (0.006 l/kg) of powder are mixed with this powder suchthat the volume percentage of liquid in the slurry is 40%. It should benoted that a sack of powder is defined by analogy with a sack of cement,one sack being 45.359 kg of mixture. In other words, 1 gps=0.0834 l ofadditive per kg of mixture.

The slurry is foamed using the same procedure, after adding surfactantsas in Slurry A, to obtain a foam quality of 40% (i.e. the volume of thefoam represented 40% of the final volume of the foamed slurry).

The compressive strength and water permeability are measured usingsamples which are left at ambient temperature and at atmosphericpressure for 48 h then in an oven at 85° C. for 5 days.

Slurry A (reference) B Porosity before 59% 40% foaming Compressive 5.93(860) 5.59 (810) strength, MPa (psi) Water permeability 0.008 1.2(Darcy)

This example demonstrates that a composition within the scope of theinvention can provide comparable compressive strength to prior artslurries while demonstrating dramatically increased permeability,despite the lower initial porosity of the slurry.

EXAMPLE 2

The properties of five slurries prepared in accordance with theinvention but foamed with different foam qualities are presented andcompared:

Basic slurry: A mixture of powders is prepared comprising 30% by volumeof haematite particles with a mean size of about 300 microns; 30% byvolume of hollow spheres with a mean size of 180 microns; 30% by volumeof class G Portland cement and 10% by volume of a Portland/slagmicro-cement with a mean size of about 3 microns. Water and apolynaphthalene sulfonate-based super-plasticizer in an amount of 0.07gallons per sack of powder (0.006 l/kg) are mixed with this powder suchthat the volume percentage of liquid in the slurry is 40%. Surfactantsare added to the basic slurry and the slurries foamed to obtain a foamqualities of 30% to 50% (a foam quality of 30% means that the foamvolume represents 30% of the final volume of the slurry).

Slurry C1 C2 C3 C4 C5 Porosity before foaming 40% 40% 40% 40% 40% Foamquality 30% 35% 40% 45% 50% Compressive strength, 8.27 6.90 5.59 4.482.83 MPa (psi) (1200) (1000) (810) (650) (410) Water permeability 0.00450.160 1.2 6.1 >12 (Darcy)

The compressive strength and water permeability are measured usingsamples which are left at ambient temperature and at atmosphericpressure for 48 h then in an oven at 85° C. for 5 days.

EXAMPLE 3

The properties of two slurries prepared in accordance with theinvention, both containing self-destructive particles but with coarseparticles of differing size and nature, are presented and compared:

Slurry D: A mixture of powders is prepared comprising 30% by volume ofhaematite particles with a mean size of about 300 microns; 30% by volumeof hollow cenospheres) with a mean size of 180 microns; 30% by volume ofclass G Portland cement and 10% by volume of a Portland/slagmicro-cement with a mean size of about 3 microns. Water and apolynaphthalene sulfonate-based super-plasticizer in an amount of 0.07gallons per sack of powder (0.006 l/kg) are mixed with this powder suchthat the volume percentage of liquid in the slurry is 40%.

Slurry E: A mixture of powders is prepared comprising 30% by volume ofcalcium carbonate particles with a mean size of about 500 microns; 30%by volume of hollow cenospheres with a mean size of 180 microns; 30% byvolume of class G Portland cement and 10% by volume of a Portland/slagmicro-cement with a mean size of about 3 microns. Water and apolynaphthalene sulfonate-based super-plasticizer in an amount of 0.07gallons per sack of powder (0.006 l/kg) are mixed with this powder suchthat the volume percentage of liquid in the slurry is 40%.

These two slurries are foamed using the same procedure, after addingsurfactants as in slurry A, to obtain a foam quality of 35% (i.e., thevolume of the foam represented 35% of the final volume of the foamedslurry).

Slurry D E Porosity before foaming 40% 40% Compressive strength 6.90(1000) 5.79 (840) Water permeability (D) 0.160 0.270

The compressive strength and water permeability are measured usingsamples which are left at ambient temperature and at atmosphericpressure for 48 h then in an oven at 85° C. for 5 days.

EXAMPLE 4

A formulation demonstrating good properties with 60% particles ofaverage size between 20 and 50 microns and 30% of large particles.

A powder blend is mixed containing 10% by volume mix portlandmicro-cement/slag, average size 3 microns, 60% class G cement and 30%hollow spheres of average size 320 microns. Water and polynaphthalenesulphonate dispersant (0.07 gallons per sack of powder blend, 0.006l/kg) are added so that the volume of liquid in the slurry is 45% andthe slurry is foamed as above to obtain a foam quality of 40%.

The slurry is heated in a sealed container for two days at 85° C. Theresultant cement has a permeability of 1.2 darcy and a compressivestrength of 910 psi.

EXAMPLE 5

A formulation demonstrating that high permeability can be obtained withtwo particle sizes only and with 40% fine particles.

A powder blend is mixed containing 40% by volume Portlandmicro-cement/slag, average size 3 microns, 30% hollow spheres of averagesize 180 microns and 30% hematite of average size 300 microns. Water andpolynaphthalene sulphonate dispersant (0.07 gallons per sack of powderblend, 0.006 l/kg) are added so that the volume of liquid in the slurryis 45% and the slurry is foamed as above to obtain a foam quality of55%.

The slurry is left in a sealed container for two days at ambienttemperature and then 5 days in an oven at 85° C. The resultant cementhas a permeability of 13 darcy and a compressive strength of 300 psi.

EXAMPLE 6

A formulation including large particle size cement as some of the largeparticles giving increased compressive strength whilst retainingsignificant permeability.

The formulation is identical to C3 above in Example 3 (40% foam, 30%hematite, 30% hollow spheres, 30% cement and 10% micro cement) exceptthat hematite is replaced by large particle cement. Formulation C3 has apermeability of 1.2 darcy and compressive strength of 810 psi. Thisformulation with hematite replaced by large particle size cement haspermeability of 0.62 Darcy but compressive strength increases to 1240psi.

EXAMPLE 7

Permeable cement can be made without large particles.

A powder blend is mixed containing 10% by volume mix Portlandmicro-cement/slag, average size 3 microns, 35% class G cement and 55%hollow spheres of average size 55 microns. Water and polynaphthalenesulphonate dispersant (0.07 gallons per sack of powder blend, 0.006l/kg) are added so that the volume of liquid in the slurry is 45% andthe slurry is foamed as above to obtain a foam quality of 40%.

The slurry is left in a sealed container for two days at ambienttemperature and then 5 days in an oven at 85° C. The resultant cementhas a permeability of 0.15 darcy and a compressive strength of 1310 psi.

Emulsion Formulations

One approach for emulsion formulations within the scope of the inventionis to make the slurry blend (particles, dispersant and water) and thenmix in surfactant and oil to give an emulsion (oil in cement phase orcement phase in oil depending on the surfactant).

EXAMPLE 8

A powder base made up with 30% cement (medium particles), 10% microcement (fine particles), 30% hollow spheres and 30% hematite (largeparticles). Water is added to be 40% by volume of the slurry and thesame dispersant is used as in the foam formulations described above.Surfactant is added at 0.04 gallons per sack of powders (0.003 l/kg) andoil is added so as to be 40% of the volume of the total emulsion system.The oil used is a linear alphaolefin. The slurries are placed in an ovenfor 2 days at 85° C.

Two surfactants are used—a sulfated ethoxylated nonyl phenol (slurry F)and an ethylene-oxide propylene oxide surfactant with a cloud point ataround 60° C. (slurry G).

Slurry F shows a compressive strength of 345 psi and permeability towater<0.1 md. Slurry G shows a compressive strength of 649 psi andpermeability to water of 2.5 md and a permeability to oil of 20 md.

EXAMPLE 9

A powder base is prepared with 30% cement, 10% micro cement, 50% hollowspheres and 10% large particle size cement. Water is added so as to be40% by volume of the slurry and a dispersant used as in the foamexamples above. Oil is added at 40%, 50% or 60% of volume of total andthe surfactant is used at 0.9% by volume of oil added. The slurries areplaced in an oven for 2 days at 85° C.

40% oil slurry: permeability to water of 4 md, compressive strength=481psi 50% oil slurry: permeability to water of 78 md and a permeability tooil of 249 md, compressive strength to water of 349 psi

60% oil slurry: permeability to water of 61 md, compressive strength=138psi

Increasing the amount of oil allows greater connectivity with other oildroplets and with hollow spheres so as to create a potentially permeablestructure. High levels of oil and surfactant can inhibit setting of thecement and hence inhibit the destruction of the hollow spheres which inturn impacts the permeability of the final cement.

EXAMPLE 10

By using a degradable surfactant the impact on development of apermeable structure as described above can be mitigated.

A powder base is made up with 30% cement, 10% micro cement, 50% hollowspheres and 10% large particle size cement and water is added so as tobe 40% by volume of the slurry and dispersant used as in the foamexamples above. Three ethoxylated castor oil surfactants are used withdecreasing ethoxy-chain lengths—surfactant A>surfactant B>surfactant C.The surfactants are used at 0.06 gallons per sack of powders (0.005l/kg). Oil is added so as to be 40% of the volume of the total emulsionsystem. The oil used is a linear alphaolefin. The fluids are placed inan oven for 2 days at 85° C.

Surfactant A system: permeability to water of 0.1 md, compressivestrength 618 psi

Surfactant B system: permeability to water of 4.5 md, compressivestrength 449 psi

Surfactant C system: permeability to water of 75 md, compressivestrength 1208 psi

EXAMPLE 11

A powder base is made up with 10% micro cement, 30% class G cement, 30%hematite, 30% hollow spheres. The powder is mixed with water anddispersant (total 40% of slurry volume) and the resulting slurry placedunder 3000 psi pressure for 10 minutes to simulate the effect of wellpressures on the hollow spheres (some may break). The slurry is thenfoamed to 40% quality and put in a sealed cell for 2 days at 85° C.After setting the initial permeability to water is 8.9 Darcy decreasingwith time and rate to 3 Darcy. The permeability to oil of the samesample is 13 Darcy increasing to 19 Darcy with time and rate. Thedifference in permeability to oil and water will allow preferentialproduction of oil from a formation producing both oil and water.

1-76. (canceled)
 77. A method of remedial treatment of voids behindcasing in a borehole, comprising: a) preparing a slurry comprising: i)an aqueous continuous phase; ii) a water-immiscible dispersed nitrogenor oil fluid phase, in an amount of from 40%-50% of the total volume ofthe slurry; and iii) a blend of solid particles comprising hydrauliccement particles and hollow particulate material, in an amount of10%-60% by volume of solid blend; b) pumping the slurry into the void;and c) allowing the cement slurry to set at a temperature such that thehollow particulate material breaks down due to the chemical and thermalenvironment in the cement, so as to leave voids which together with thedispersed phase result in a set cement having a permeability of at least1 Darcy.
 78. The method of claim 77, wherein the solid blend comprises acombination of at least two of a fine particulate material havingparticle sizes in the range 0.1-10 micron, a medium particulate materialhaving particle sizes in the range 20-60 micron and a coarse particulatematerial having particle sizes in the range 100-800 micron.
 79. Themethod of claim 77, wherein the slurry comprises a liquid fraction inthe range of 38%-50% by volume of the slurry when the dispersed phase isa gas.
 80. The method of claim 77, wherein, when the dispersed phasecomprises oil, the slurry comprises 38%-50% by volume of an aqueousphase and 40% 60% by volume of the total of oil.
 81. The method of claim77, wherein the hydraulic cement is Class G Portland cement and thehollow particulate material comprises cenospheres.
 82. The method ofclaim 77, wherein the cement is allowed to set at a temperature of atleast 85° C.
 83. A method of completing a well, comprising: a) preparinga slurry comprising: i) an aqueous continuous phase; ii) awater-immiscible dispersed nitrogen or oil fluid phase, that amounts tofrom 40%-50% of the total volume of the slurry; and iii) a blend ofsolid particles comprising hydraulic cement particles and a hollowparticulate material, in an amount of 10%-60% by volume of solid blend;b) pumping the slurry into an annulus around a casing, screen or slottedliner placed in a well; and c) allowing the cement slurry to set at atemperature such that the hollow particulate material breaks down due tothe chemical and thermal environment in the cement, so as to leave voidswhich together with the dispersed phase result in a set cement having apermeability of at least 1 Darcy.
 84. The method of claim 83, whereinthe solid blend comprises a combination of at least two of a fineparticulate material having particle sizes in the range 0.1-10 micron, amedium particulate material having particle sizes in the range 20-60micron and a coarse particulate material having particle sizes in therange 100-800 micron.
 85. The method of claim 83, wherein the slurrycomprises a liquid fraction in the range of 38%-50% by volume of theslurry when the dispersed phase is a gas.
 86. The method of claim 83,wherein when the dispersed phase comprises oil, the slurry comprises38%-50% by volume of an aqueous phase and 40%-60% by volume of the totalof oil.
 87. The method of claim 83, wherein the hydraulic cement isClass G Portland cement and the hollow particulate material comprisescenospheres.
 88. The method of claim 83, wherein the cement is allowedto set at a temperature of at least 85° C.
 89. A method of preventingsand production from an underground formation into a well, comprising:a) preparing a slurry comprising: i) an aqueous continuous phase; ii) awater-immiscible dispersed nitrogen or oil fluid phase, that amounts tofrom 40%-50% of the total volume of the slurry; and iii) a blend ofsolid particles comprising hydraulic cement particles and a hollowparticulate material, in an amount of 10%-60% by volume of solid blend;b) pumping the slurry into a region adjacent the formation; and c)allowing the cement slurry to set at a temperature such that the hollowparticulate material breaks down due to the chemical and thermalenvironment in the cement, so as to leave voids which together with thedispersed phase result in a set cement having a permeability of at least1 Darcy.
 90. The method of claim 89, wherein the solid blend comprises acombination of at least two of a fine particulate material havingparticle sizes in the range 0.1-10 micron, a medium particulate materialhaving particle sizes in the range 20-60 micron and a coarse particulatematerial having particle sizes in the range 100-800 micron.
 91. Themethod of claim 89, wherein the slurry comprises a liquid fraction inthe range of 38%-50% by volume of the slurry when the dispersed phase isa gas.
 92. The method of claim 89, wherein, when the dispersed phasecomprises oil, the slurry comprises 38%-50% by volume of an aqueousphase and 40%-60% by volume of the total of oil.
 93. The method of claim89, wherein the hydraulic cement is Class G Portland cement and thehollow particulate material comprises cenospheres.
 94. The method ofclaim 89, wherein the cement is allowed to set at a temperature of atleast 85° C.
 95. A method of replacing eroded cement behind a casing ina well, comprising: a) preparing a slurry comprising: i) an aqueouscontinuous phase; ii) a water-immiscible dispersed nitrogen or oil fluidphase, that amounts to from 40%-50% of the total volume of the slurry;and iii) a blend of solid particles comprising hydraulic cementparticles and a hollow particulate material, in an amount of 10%-60% byvolume of solid blend; b) pumping the slurry into the region of theeroded cement; and c) allowing the cement slurry to set at a temperaturesuch that the hollow particulate material breaks down due to thechemical and thermal environment in the cement, so as to leave voidswhich together with the dispersed phase result in a set cement having apermeability of at least 1 Darcy.
 96. The method of claim 95, whereinthe solid blend comprises a combination of at least two of a fineparticulate material having particle sizes in the range 0.1-10 micron, amedium particulate material having particle sizes in the range 20-60micron and a coarse particulate material having particle sizes in therange 100-800 micron.
 97. The method of claim 95, wherein the slurrycomprises a liquid fraction in the range of 38%-50% by volume of theslurry when the dispersed phase is a gas.
 98. The method of claim 95,wherein, when the dispersed phase comprises oil, the slurry comprises38%-50% by volume of an aqueous phase and 40%-60% by volume of the totalof oil.
 99. The method of claim 95, wherein the hydraulic cement isClass G Portland cement and the hollow particulate material comprisescenospheres.
 100. The method of claim 95, wherein the cement is allowedto set at a temperature of at least 85° C.
 101. A method of producingoil from an oil and water producing formation comprising: a) preparing aslurry comprising: i) an aqueous continuous phase; ii) awater-immiscible dispersed nitrogen or oil fluid phase, that amounts tofrom 40%-50% of the total volume of the slurry; and iii) a blend ofsolid particles comprising hydraulic cement particles and a hollowparticulate material, in an amount of 10%-60% by volume of solid blend;b) pumping the slurry into or adjacent to a producing formation; and c)allowing the cement slurry to set at a temperature such that the hollowparticulate material breaks down due to the chemical and thermalenvironment in the cement, so as to leave voids which together with thedispersed phase result in a set cement having a permeability of at least1 Darcy but lower than the oil permeability of the set cement, so as toproduce fluid from the reservoir with an oil water ratio higher thanwould normally be possible from an untreated formation.
 102. The methodof claim 101, wherein the solid blend comprises a combination of atleast two of a fine particulate material having particle sizes in therange 0.1-10 micron, a medium particulate material having particle sizesin the range 20-60 micron and a coarse particulate material havingparticle sizes in the range 100-800 micron.
 103. The method of claim101, wherein the slurry comprises a liquid fraction in the range of38%-50% by volume of the slurry when the dispersed phase is a gas. 104.The method of claim 101, wherein, when the dispersed phase comprisesoil, the slurry comprises 38%-50% by volume of an aqueous phase and40%-60% by volume of the total of oil.
 105. The method of claim 101,wherein the hydraulic cement is Class G Portland cement and the hollowparticulate material comprises cenospheres.
 106. The method of claim101, wherein the cement is allowed to set at a temperature of at least85° C.